Using binary system to solve for the Mass

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Using the fact that Cyg X-1 is in a Binary System to find its
Mass

In general, it is not possible to directly determine the mass of a single
star from its light alone (though this may soon change as instruments are
becoming sensitive enough to measure the second order effect in the
relativistic Doppler shift, which can provide the mass). But in the case of
a binary system, the mass of the stars can usually be determined, if
certain assumptions are made.

Cygnus X-1 is part of a binary, or double star system. Its companion star, HDE 226868, is a BO Supergiant.